4 Contents iii List of Tables Chapter 2 Table 2.1 Environmental characteristics of the study sites Chapter 3 Table 3.1 Major biotic variables (Minimum, Maximum, Mean ± 1 S.D.) for invertebrate assemblages and the organic content of the stones sampled Table 3.2 Physical and chemical conditions measured during drift sampling Table 3.3 Biotic and abiotic drift variables (Mean ± 1 S.D.) measured at the sample sites Chapter 4 Table 4.1 Physical, chemical, geographical and geomorphological characteristics at the sampling stations along the Ilm profile Table 4.2 The four categories used to rank the invertebrates by their abundance.. 68 Table 4.3 The eight categories, their codes and rank scores used for analysis Table 4.4 The seven flow preference categories, their codes and a description of the categories used for analysis Table 4.5 The eight habitat-preference categories used in this study Table 4.6 Invertebrate taxon richness at seven sampling stations along the longitudinal profile of the Ilm during Chapter 5 Table 5.1 The leaf material fractions analyzed in this study Table 5.2 Survival (%) of G.pulex and B. rhodani in the experiments Table 5.3 The amount of particles generated during breakdown experiments relative to leaf mass initially added and relative to leaf mass loss Table 5.4 Chemical and isotopical composition of the leaf materials and the changes observed during leaching experiments... 99

5 iv Contents List of Figures Chapter 2 Figure 2.1 Maps of Germany, Thuringia and a schematic view to the study sites (SS1 to SS4), the dam is located within the town Stadtilm (11 05 E, N, 360 m a.s.l.) Figure 2.2 Map of the Ilm section, containing the four study sites Figure 2.3 A picture of the dam investigated in this study. (View from the site immediately downstream) Figure 2.4 Discharge at the permanent gauge Gräfinau-Angstedt 10 km upstream from the study area from 1. January 2000 to 30. October Figure 2.5 Schematic view of the Hess-Sampler used in this study Figure 2.6 Means (± 1 S.D.) for invertebrate density (A), biomass (B), Simpson`s index of diversity (C), expected taxon richness (D), number of taxa (E) and number of invertebrate eggs (F) during four seasons at the four sampling sites Figure 2.7 Relative abundance of functional feeding groups based on the averaged abundance from all samples Figure 2.8 Means (± 1 S.D.) for total POM (A,B), CPOM (C,D) and FPOM (E,F) contents expressed as (g m -2 ) and (mg g -1 ) during four seasons at the four sampling sites Figure 2.9 RDA triplot showing the samples (circles), the benthic invertebrate taxa (down-triangles) and the environmental variables (arrows) Figure 2.10 RDA plots after classification of the samples according to sample site (plot A), to sampling season (plot B) and to macrohabitat type (pool / riffle) (plot C) Figure 2.11 The relation observed between the averaged daily discharge prior to sampling and the Spearman Rank Correlation Coefficient between the invertebrate density (Ind. g -1 ) and the total amount of POM (mg g -1 ) Chapter 3 Figure 3.1 A schematic view of the Drift-Sampler Figure 3.2 A picture of the Drift Sampler in the stream Figure 3.3 (A) Sample-based rarefaction curves of invertebrate assemblages based on corresponding numbers of randomized accumulated samples. Taxon richness appears to be similar for the three sample sites. (B) Sample-based rarefaction curves after re-scaling the x-axis from samples to invertebrates (individuals) Figure 3.4 Ordination plot of a principal component analysis based on the abundance of 32 invertebrate taxa in 72 samples investigated in this study

6 Contents v Figure 3.5 Two proposed successional directions for invertebrate assemblages on stones. The direction of assemblages succession after disturbance seems affected by the position of stone in the stream (light dark mosaic) and the colonization and growth of filamentous algae (like Cladophora sp.) Chapter 4 Figure 4.1 Maps of Thuringia and a detailed view of the Ilm stream Figure 4.2 Ordination plot of a correspondence analysis based on the on rank - abundance of 117 invertebrate taxa (Ephemeroptera, Trichoptera, Chironomidae and Coleoptera) at seven sampling stations at the longitudinal profile of the Ilm during Figure 4.3 Functional feeding group composition along the stream profile of the Ilm during 1992/ Figure 4.4 Biocoenotic zonation at the Ilm, indicated by the zonational index (Z). 74 Figure 4.5 Changes in flow preference composition of invertebrate communities along the stream profile of the Ilm during 1992/ Figure 4.6 Changes in habitat preference composition of invertebrate communities along the stream profile of the Ilm during 1992/ Chapter 5 Figure 5.1 Schematic view of one experimental unit (plexiplas chamber) used in the experiments Figure 5.2 A view to the environmental container Figure 5.3 Relative growth rates (Mean ± 1 S.E.) for three body size parameters (length, side area and dry weight) of Gammarus pulex fed with leaf material in the experiments Figure 5.4 Relative growth rates (Mean ± 1 S.E.) for three body size parameters (length, head width and estimated dry weight) of Baetis rhodani fed with leaf material in the experiments Figure 5.5 Particle size composition in the leaf treatments after the experiments. 97 Figure 5.6 Stable carbon (δ 13 C) and nitrogen (δ 15 N) composition of leaf materials and invertebrates before and after breakdown experiments. 102 Figure 5.7 Figure 5.8 Changes in δ 13 C ( ) (A & B), δ 15 N ( ) (C & D) and AFDM (%) (E & F) in the leaf material of the L1 treatments during breakdown Changes in carbon contents (%) (A & B), nitrogen contents (%) (C & D) and C/N ratios (E & F) in the leaf material of the L1 treatments during breakdown

8 1. General Introduction 1 1. Introduction Chapter 1 Streams are indispensable components of the global hydrologic cycle and they are essential in global biochemical processes. It has long been recognized that streams not only serve as simple transport channels, today they are considered as complex ecosystems that intensively exchange energy and matter with the surrounding terrestrial ecosystems. The River Continuum Concept (RCC) describes stream ecosystems as a continuous series of physical gradients and associated biotic adjustments (Vannote et al. 1980). The concept largely focused on the interaction of stream organisms with their habitat and food resources suggesting that longitudinal changes in structural and functional characteristics of the stream community reflects longitudinal changes in the availability of habitats and of various forms of organic matter along a stream system. In many headwater streams the riparian vegetation is the primary source of organic matter and also limits the autochthonous primary production by shading. Therefore, most of the organic matter available to this stream type is of allochthonous origin. For instance, Fisher and Likens (1973) found that the input of tree leaves accounted for 99% of the energy input to a headwater stream. After entering the stream two things happen to the allochthonous material: breakdown and downstream transport. Breakdown is the result of the combined action of physical, chemical and biological processes and occurs in three phases: leaching, microbial colonization (conditioning) and fragmentation by physical forces and invertebrate feeding (Suberkropp 1998). The shredder invertebrates which are specialized in feeding on coarse particulate organic matter (CPOM) are a proportionally more common functional feeding group in headwater streams and play a crucial role in stream energy dynamics by converting coarse particulate organic matter (CPOM) into fine particulate organic matter (FPOM), which is then transported downstream (Winterbourn & Davis 1976, Wallace & Webster 1996, Hieber & Gessner 2002). Beside the particle size also the nutritional quality of the organic material changes during detritus processing. With increasing stream size, autochthonous primary production (aquatic macrophytes & periphyton) and fine particulate organic matter (FPOM) supplied from upstream processing of coarse particulate organic matter (CPOM) become important as energy sources for the stream communities. Therefore, the invertebrate communities in the middle reaches of a stream system will be dominated by grazers and collectors (Vannote et al. 1980). Further downstream in the largest reaches of a stream system planktonic algae will become the dominant primary producer and organic matter resources will be present as fine particulate organic matter (FPOM) and ultra-fine

9 2 1. General Introduction particulate organic matter (UPOM). As a consequence filtering collectors (planktonic & benthic) will be the dominant functional feeding group in these reaches. The unidirectional flow of water is the major physical force and the controlling ecological factor (Schönborn 1992) that causes a strong linkage of downstream reaches with the upstream reaches (Fisher & Likens 1973). Breakdown and downstream transport occur simultaneously in streams. The spiraling concept (Webster & Patten 1979, Newbold et al. 1982, Elwood et al. 1983) describes the spatio-temporal dynamics of nutrients and organic carbon. The cycles of organic matter and nutrients are stretched into spirals due to the continuous, unidirectional flow of the water. For nutrients a cycle is completed when a nutrient atom has been taken up by an organism from a dissolved available state (inorganic), passed through the food chain, and returned to a dissolved available state for reutilization (Newbold et al. 1982). The spiraling length and spiraling time, defined as the spatial distance and the time required for a complete cycle are used to describe the spiraling of nutrients. Because of the permanent carbon dioxide (inorganic carbon) exchange between air and water, carbon spiraling is more difficult to conceptualize. The turnover length and turnover time are used as measures for the spatio-temporal extension of carbon cycles and are defined as the distance and time traveled by a carbon atom during its residence in the stream in an organic form. But they do not refer to the complete spiral of a carbon atom. Short spiraling length and time indicate efficient recycling of resources. The spiraling of nutrients and organic matter is a function of physical and biological processes. The downstream transport and the retention of organic matter are most important in determining carbon spiraling length. Both are critical functions in stream ecosystems, because they control the loss of nutrients, particle-associated energy and connect upstream processes with downstream ones (Hall et al. 1996). In contrast, carbon spiraling time seems mainly determined by biological processes, such as respiration and invertebrate feeding activities. The continuous changes of many physical and biological processes along a stream continuum (Vannote et al. 1980) seems to cause a continuous change in the spiraling of organic matter from upstream to downstream reaches along a stream (Minshall et al. 1983). Numerous anthropogenic disturbances alter stream ecosystems (Resh et al. 1988, Covich 1993) and impoundment is an very important one. Dams are ubiquitous structures in many stream systems throughout the world. Large storage dams alter the discharge and the temperature regimes, hydraulic characteristics, substrate composition and channel morphology and as a consequence the structure of stream communities. Many of these alterations can persist for large distances downstream. This type of dam furthermore alters the river continuum by disrupting the spiraling of

10 1. General Introduction 3 resources (nutrients and organic matter) (Ward and Stanford 1983) and disconnecting upstream and downstream reaches (Pringle 1997) resulting in declines in biodiversity and the alteration of natural stream food webs (Power et al. 1996, Wootton et al. 1996). Based on the RCC, Ward and Stanford (1983, 1995) developed the serial discontinuity concept (SDC) that considers the alterations caused by large, deep-release storage dams. The SDC describes the local effects of large storage dams, their consequences for the entire stream system and further considers the effects of multiple impoundments. Beside general impacts on physical, chemical and biological conditions, the barrier effect is a further serious one that can result from damming. Dams may reduce the hydrological connectivity by preventing or impeding the migration of organisms throughout the stream system (Pringle 2003), resulting in fragmentation of habitat and isolation of populations (Pechlaner 1986, Drinkwater & Frank 1994, Winston et al. 1991, Marchant & Hehir 2002). The barrier effect seems evident for migratory fish species (Lewis 1991, Mills 1989, Morita & Yamamoto 2002). However, up today most studies focused only on economically important populations of fish (migratory salmonid fishes) and comparably little is known about biota of less economic importance. The effect is especially for aquatic invertebrates by far less clear (Pringle 2003). In contrast to large dams today there is relatively little information on the ecological impacts of smaller dams. Such small often run of river dams with a hydraulic head < 5 meters (low head) and small impounded areas of < 20 hectare, are generally much more numerous than large storage dams in Central European stream systems. This type of dam does often not substantially alter the natural discharge regime, but influence local flow velocity patterns, sediment composition (Magilligan & Nislow 2001, Stanley et al. 2002), particulate organic matter (POM) budgets and CPOM/FPOM ratios (Gore 1994, Wagner 2003). Also the impoundment of these structures create distinct physical conditions in comparison to free-flowing natural reaches (Baekken et al. 1981b, Stanley et al. 2002), but chemical conditions are altered to a much lesser extent (Baekken et al. 1981a). The ecological consequences of small low head dams are poorly understood (Benstead et al. 1999, Hart et al. 2002, Poff & Hart 2002) and comparably little scientific interest has been laid on this kind of human impact to stream systems. The knowledge about the ecological impacts of dams today based to a large extent on studies of large storage dams (>15 m), but most of the dams which are being removed, enclosed or reconstructed (fish passes & -ladders) by the regional stream managers in Europe at present, are small low-head dams ( 5 m). Underlying this fact there is a pressing need for studies about ecological responses of dam impact across a variety of dam sizes and operational types (Hart et al. 2002).

11 4 1. General Introduction The aim of the present thesis is to investigate the impacts of a small low-head dam on invertebrate communities and particulate organic matter standing stocks in a headwater stream in Thuringia/Germany. It was hypothesized that invertebrate communities and POM standing stocks are altered in stream reaches close to the dam. It was furthermore assumed that the impacts of the dam are locally restricted and that the type, the magnitude and the spatial extension of the impacts are not comparable to those caused by large storage dams. Based on the facts, statements and assumptions mentioned above, the following questions arise: Does the dam alter the invertebrate communities and if it does, are the alterations extended downstream? The following attributes are used to investigate these questions: (a) Invertebrate abundance (number of invertebrates & biomass) (b) Diversity (taxon richness & evenness) (c) Community composition (taxonomic & functional) Does the dam alter the distribution and composition of POM and if it does, are these alterations extended downstream? The investigated attributes are: (d) POM standing stocks (e) CPOM/FPOM ratios Is there a barrier effect? The investigated attributes are: (f) Abundance, diversity & composition of invertebrate assemblages on stones (g) Abundance, diversity & composition of drifting invertebrates Furthermore the effects of multiple impoundments along the longitudinal profile of the regulated headwater stream Ilm on the zonational distribution of aquatic invertebrates are investigated. In order to understand food webs and food web alterations caused by anthropogenic disturbances a detailed knowledge about the links between consumers and resources is indispensable. Therefore, an experimental laboratory study investigates the complex interaction between resources (POM) and benthic invertebrate consumers.

12 1. General Introduction 5 Thesis structure In the first chapters the effects of a single small low-head dam on invertebrate communities and POM storage are investigated. In order to detect the impact of the dam various structural and functional attributes of the invertebrate communities (abundance, diversity, functional composition), the contents of POM in the sediment and the particle size composition of POM were measured at two dam sites and at two natural stream sites (Chapter 2). The two latter sites were selected according to structural attributes of the stream channel, indicating nearly natural conditions. The sites provide a baseline of spatial and temporal variation in invertebrate community variables and POM contents within unregulated stream reaches. Additionally in Chapter 2 the distribution of particulate organic matter (POM) is related with abundance (number & biomass) and composition of invertebrate assemblages in order to examine the spatial relation between particulate organic matter and benthic invertebrates. The barrier effect is examined in Chapter 3. Two approaches are used to explore this effect: At first invertebrate assemblages on stone surfaces are compared between a reach immediately downstream of a dam and two natural reference sites. It was assumed that the investigated low-head dam affects invertebrate colonization by alteration of downstream drift and prevention of upstream movements. As a consequence, invertebrate assemblage composition immediately downstream of the dam should differ from those of free flowing natural reaches. In order to support the results and interpretations gained from the first study part, the downstream drift of benthic invertebrates is measured in the second part of this study. It was hypothesized that the impoundment traps invertebrates from downstream drift, resulting in differences in the invertebrate drift (density & diversity) within the impoundment, but also immediately downstream of the dam, both in comparison to free flowing natural reaches. The longitudinal zonation of aquatic invertebrates, a gradual change of invertebrate community structure along stream systems, is a well known characteristic in undisturbed stream ecosystems. Little is known about the effects of multiple impoundments of small dams to stream systems. Therefore the effects of multiple impoundments along the longitudinal profile of a regulated headwater stream on the zonational distribution of aquatic invertebrates are investigated in Chapter 4. For this purpose the distribution of invertebrates along the stream gradient was assessed using four published data sets and compared with patterns suggested by basic concepts. Understanding food webs and food web alterations caused by anthropogenic disturbances are central topics in ecology. To achieve this goal it is necessary (1) to

13 6 1. General Introduction determine the sources of organic matter that provide energy and nutrients to the heterotrophs as well as their relative importance and (2) to determine the trophic pathways through which the energy of organic matter resources is transferred within food webs. Stream invertebrates are central components in stream food webs. They are essential to stream nutrient cycling by consuming and transforming organic matter and they are important in transferring energy to higher trophic levels. The functional classification of stream invertebrates has enhanced the understanding of stream nutrient cycling and trophic interactions, but the usage of functional feeding groups as trophic guilds has been criticized because they do not appropriately reflect resource utilization. The ratios of stable isotopes of nitrogen (δ 15 N) and carbon (δ 13 C) are increasingly used as natural-abundance tracers to reconstruct diets and to estimate the trophic position of the animals. This method seems useful to trace the flow of energy through the ecosystem and has been used successfully to detect food web alterations caused by human disturbances. However, there have been many field studies in which it was difficult to discern the trophic structure on the basis of isotope data. Gannes et al. (1997) argued that a correct interpretation of stable isotope data will be achieved only if the collection of field data is accompanied by laboratory experiments. Therefore, Chapter 5 focuses on the utilization of leaf resources by two benthic invertebrate species: Gammarus pulex L. (Crustacea; Amphipoda) and Baetis rhodani Pictet (Insecta, Ephemeroptera). Laboratory experiments were designed to measure utilization of leaf resources by Gammarus pulex and Baetis rhodani and further to monitor the principal changes in chemical and isotopical composition of leaf material during breakdown.

14 2. The effects of a small dam on invertebrate communities and particulate organic matter storage 7 Chapter 2 2. The effects of a small dam on invertebrate communities and particulate organic matter (POM) 2.1 Introduction Stream regulation by damming is known to alter physical, chemical and biological conditions in streams (Ward & Stanford 1979, Ward & Stanford 1983 & 1995, Petts 1984, Gore 1994, Collier et al. 1996). To date there is a large body of literature documenting the profound effects of large storage dams. Such dams are essential for power supply, flood control and water storage. The regulation by these dams has substantial economic benefits, but there are the costs of fundamental alterations of stream systems. Many physical and chemical conditions are altered continuously within the storage reservoirs leading to strong response in biological communities there and in downstream sections, but depending on the position of the dam at the longitudinal stream profile (Ward & Stanford 1979). Especially the regulation of release affects the overall discharge regime of the stream (Brookes 1994, Boulton & Brock 1999). Small run of river dams with a hydraulic head < 5 meter (low head) and small impounded areas of < 20 hectare, are much more numerous than large storage dams in Central European streams and rivers. This type of dam does not substantially alter the natural discharge regime, but influence flow velocity patterns, sediment composition (Magilligan & Nislow 2001, Stanley et al. 2002), particulate organic matter budgets and the ratio between coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) (Gore 1994, Wagner 2003). Also the impoundment of these structures create distinct physical conditions in comparison to free-flowing natural reaches (Baekken et al. 1981b, Stanley et al. 2002), but chemical conditions are altered to a much lesser extent (Baekken et al. 1981a). The ecological consequences of small low head dams are poorly understood (Benstead et al. 1999, Hart et al. 2002, Poff & Hart 2002) and comparably little scientific interest has been laid on this kind of human impact to stream systems. Whereas the knowledge about the ecological impacts of dams today is based, to a great extent, on studies of large storage dams (> 15 m), most of the dams which are being removed, enclosed or reconstructed (fish passes & -ladders) by the regional stream managers in Europe at present, are small low-head dams ( 5 m). Underlying this fact there is a pressing need for studies about ecological responses of dam impact across a variety of dam sizes and operational types (Hart et al. 2002). The objective of this study is to examine the effects of one small low-head dam on benthic invertebrate communities and particulate organic matter (POM).

15 8 2. The effects of a small dam on invertebrate communities and particulate organic matter storage Both particulate organic matter and invertebrate consumers are essential to the flow of energy in stream ecosystems. Many headwater streams are energetically dependent on the allochthonous organic matter (Fisher & Likens 1973, Cummins 1974) and POM standing stocks mostly exceed autochthonous primary production. Downstream transport and retention of particulate organic matter are critical functions in stream systems, because they control the POM standing stocks available to heterotrophs. Aquatic invertebrates contribute significantly to nutrient cycling and the turnover of organic material. Changes in detritus quality and quantity greatly influence community composition and energy budgets in streams (Vannote et al. 1980, Wallace et al a). This study aims to describe and to compare invertebrate communities and POM standing standing stocks of stream reaches close to a dam with those of natural reference reaches upstream and downstream of a small low-head dam. The dam is considered as a man made semi-experimental system that possibly disrupts transport and storage of particulate organic matter resulting in remarkable alteration of invertebrate communities. It was hypothesized that invertebrate community attributes [abundance (number of individuals & invertebrate biomass), diversity and composition] and POM standing stocks are altered in stream reaches close to the dam. It was furthermore assumed that impacts of the dam are locally restricted and that the type, the magnitude, and the spatial extension of the impacts are not comparable to those caused by large storage dams. The following set of specific hypotheses was formulated: (I) Invertebrate community Invertebrate abundance (number & biomass) is higher in stream reaches close to the dam, but there are no far reaching downstream effects: Impoundment >DR down >(NR down = NR up ) * Invertebrate diversity is lower in stream reaches close to the dam, but the effect is not extended downstream: (NR up = NR down ) > DR down > Impoundment * Community composition (taxonomic & functional) is altered in stream reaches close to the dam, but the effect is not extended downstream: (NR up ~ NR down ) ~ DR down ~ Impoundment *

16 2. The effects of a small dam on invertebrate communities and particulate organic matter storage 9 (II) Particulate organic matter POM standing stocks are higher in stream reaches close to the dam, but the effect is not extended downstream: Impoundment > DR down > (NR up = NR down ) * CPOM/FPOM ratio is lower in stream reaches close to the dam, but the effect is not extended downstream: (NR up = NR down ) > Impoundment DR down * * Abbreviations & Presuppositions: NR up - natural reach upstream, NR down - natural reach downstream, DR down - dam reach immediately downstream of the dam, Impoundment - stream reach within the impoundment of the dam; [~ (highly similar) to ~ (least similar)]; The natural reaches are in close spatial proximity to the dam (few kilometers) and both are highly similar in their abiotic characteristics. Most of the assumptions involved in the above hypotheses, concerning the alterations at the low-head dam, are highly similar to the patterns commonly found at large storage dams, but the hypotheses differ in that no profound and far-reaching downstream effects are assumed for the investigated small low-head dam. Additionally particulate organic matter (POM) was related with invertebrate abundance (number & biomass) in order to examine the influence of POM on the distribution of benthic invertebrates.

17 10 2. The effects of a small dam on invertebrate communities and particulate organic matter storage 2.2 Methods Study area The study was conducted in the 3 rd order (Strahler 1957) hard water stream Ilm in Thuringia (Germany). The stream has a catchment area of approximately 1035 km 2 (Krey 1995). Average annual precipitation within this region ranges between mm m -2 year -1 leading to an average annual discharge of 83 x 10 6 m 3 year -1. The stream has an average slope of 3.16% (spring: 500 m a.s.l.; mouth: 115 m a.s.l.). Over its entire length of 137 km, 56 small low head dams are continuously distributed (average distance from another 2.30 km). All these dams are low-head, run-of- river dams with a hydraulic head < 5 meters (min max meters) and small impounded areas of < 3 hectare. Four sampling sites were selected within a five kilometer long section located close to the town Stadtilm (11 05 E, N, 360 m a.s.l.) in the metarithric zone of the stream (Figure 2.1). The four sampling sites were denoted as SS1, SS2, SS3, and SS4. Germany 40 km Thuringia Jena Stadtilm N flow SS 1 SS 2 SS 3 SS 4 flow reference site upstream dam site impoundment dam site downstream reference site downstream Figure 2.1: Maps of Germany, Thuringia and a schematic view to the study sites (SS1 to SS4, the dam is located within the town Stadtilm, E, N, 360 m a.s.l.).

18 2. The effects of a small dam on invertebrate communities and particulate organic matter storage 11 All the sites consisted of a marked 100 meter reach. Two sites very close to a small 2.10 m high run-off-river dam and two reference sites one ~ 1.65 km downstream and the other ~ 3.28 km upstream of the dam were chosen (Figure 2.2). SS1 SS2 SS3 SS4 flow flow N 1 km Figure 2.2: Map of the Ilm section, containing the four study sites. The partly adjustable dam (Figure 2.3) is representative for low head dams of the Ilm and was built more than 100 years ago. The reaches at the reference sites (upstream of the dam = SS1 & downstream of the dam = SS4) contained a typical pool-riffle sequence. The site (SS3), approximately 100 meter downstream of the dam, formed a continuos riffle. The second dam site (SS2) was 160 m upstream of the dam within the impoundment and is a pool site entirely. At all four sampling sites the vegetation on the banks was dominated by willow (Salix sp.), maple (Acer platanoides L.), ash (Fraxinus execlsior L.), alder (Alnus glutinosa L.), and poplar (Populus tremula L.). The site upstream of the dam (SS1) and the site close downstream of the dam (SS3) were only sparsely shaded by trees, whereas the impounded site (SS2) and the reference site downstream of the dam (SS4) were heavily shaded. The distance between the upstream and downstream reference site was chosen as small as possible to insure that at mean discharge levels a water column will pass all four sites within one day. The choice of this small spatial scale insures that longitudinal changes in most chemical

19 12 2. The effects of a small dam on invertebrate communities and particulate organic matter storage factors and hydrology were negligible between the sampling sites. Furthermore, environmental variables such as macroclimate, geology, and other catchment features can be assumed to be similar for the study reaches. Figure 2.3: A picture of the dam investigated in this study. (View from the site immediately downstream). Discharge was obtained from the nearest permanent hydrograph station (Gräfinau- Angstedt) located approximately 10 km upstream of the dam. At this station, the averaged annual discharge (long-term mean from 1923 to 2003) approximates 2.44 m 3 s -1 (Staatliches Umweltamt Erfurt, unpubl. data). The averaged annual discharge ( ) measured at the investigated dam was 3.30 m 3 s -1. Six small tributaries, occurring between the two locations seem to be responsible for the observed difference. The monthly mean discharges ( ) at the gauge in Gräfinau- Angstedt to the monthly mean discharges at Stadtilm ( ) were used in order to construct a relationship between discharges at the two points on the stream. The relationship was fitted by a regression line: Q Stadtilm = x Q Gräfinau-Angstedt ; R 2 = 0.983; P < 0.001; n=12 where Q Stadtilm = discharge at Stadtilm and Q Gräfinau-Angstedt = discharge at Gräfinau- Angstedt in m 3 s -1. This equation was used to estimate the discharge regime in the study region. During the studied period (April 2001 January 2002) no large spates occur (Figure 2.4). The mean discharge during the study period measured at the

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